The molecular mechanisms whereby the immune tolerance to self tissues is maintained are not well understood. T cell activation and tolerance is regulated by the innate immune system, importantly through co-stimulatory molecules. Activation of T cells in the absence of positive cositmulation (CD28 and ICOS) results in their tolerance and anergy, which is the main mechanism for peripheral tolerance of T cells in resting states. On the other hand, other costimulatory pathways, such as CTLA4 and PD-1 have been discovered to inhibit T cell activation. Thus, members of the B7 costimulator family, through their receptors in the CD28 family, play essential roles in determining T cell activation or self-tolerance. We have recently identified and characterized two new B7-like inhibitory molecules in mouse- B7-H3 and B7S1. Blocking antibodies against them exacerbated experimental allergic encephalomyelitis (EAE) disease in vivo. BTNL2 is a butyrophilin molecule we recently characterized that binds to activated T cell and inhibits their proliferation. Despite a large number of T cell inhibitors identified in the recent years, their specific physiological function in regulation of immune tolerance has not been well understood. In this application, we propose to compare the actions by B7-H3, B7S1 and BTNL2 in immune tolerance, specifically by analyzing their roles in peripheral tolerance and in pathogenesis of autoimmune disease models. First, we characterize the function of B7-H3 and B7S1 in peripheral tolerance mechanisms. Secondly, we will study the roles of B7- H3 and B7S1 in autoimmune disease models. Lastly, we will examine the expression and function of BTNL2. These studies will greatly improve our understanding of these novel costimulatory molecules in immune tolerance regulation.